1. Field of the Invention
The present invention relates to a belt driving device for controlling the driving speed of a driving source of a driving/rotating body for endlessly moving a belt member by detecting the speed variation pattern during one rotation of the belt member. Furthermore, the present invention relates to an image forming apparatus such as a copier, a fax machine, and a printer, that is provided with such a belt driving device.
2. Description of the Related Art
Conventionally, such an image forming apparatus is known, as described in patent document 1. The image forming apparatus includes a transfer unit for endlessly moving an endless intermediate transfer belt acting as a belt member, which is stretched around a driving roller and plural subordinate rollers. The image forming apparatus includes four separate photoconductors corresponding to yellow, magenta, cyan, and black (hereinafter, “Y, M, C, and K”) as image carriers. The Y, M, C, and K toner images separately formed on the corresponding photoconductors are sequentially superposed on the intermediate transfer belt by the transfer unit, to form a full-color image. In another example of an image forming apparatus, instead of using an intermediate transfer belt, a sheet conveying belt for conveying a recording sheet held on its endlessly moving surface may be used. The Y, M, C, and K toner images formed on the corresponding Y, M, C, and K photoconductors are directly transferred and superposed on the recording sheet on the sheet conveying belt. The method performed by these image forming apparatuses is referred to as a tandem method, in which plural image carriers are provided, toner images of colors are formed on the corresponding image carriers, and the toner images are transferred and superposed on the surface of a belt member or on a recording sheet on the belt member.
In a tandem-type image forming apparatus, when the speed of the belt member varies, the toner images of the respective colors, which are transferred and superposed on the belt member or the recording sheet, may be displaced from one another, thereby causing so-called color shift (displacement of colors). One of the factors causing the speed variation of the belt member may be the inconsistency in the thickness of the belt member (belt thickness inconsistency) in the circumferential direction. When a relatively thick part of the belt member is wound around the driving roller which drives the belt member, the belt moving speed increases. Conversely, when a relatively thin part of the belt member is wound around the driving roller which drives the belt member, the belt moving speed decreases. In this manner, the speed varies during one rotation of the belt member. A belt member that has been manufactured by a centrifugal molding technique is likely to have an inconsistent thickness due to the eccentricity of the die, in which the thickest portion and the thinnest portion may have a phase difference of 180° within one rotation of the belt. The speed variation during one rotation of the belt depicts a sine curve corresponding to one period.
In the image forming apparatus described in patent document 1, an encoder is provided for the subordinate rollers that are rotated by the endless movement of the intermediate transfer belt that is stretched around the subordinate rollers. Based on output pulses from this encoder, the endless movement speed of the belt is detected. The detected endless movement speed is stored in predetermined periods to determine the speed variation pattern during one rotation of the belt. The driving speed of the driving motor which is the driving source of the driving roller is adjusted so that the driving speed has an opposite phase to that of the waveform of the speed variation pattern. In this manner, the driving speed of the belt is adjusted, to drive the belt at various speeds in such a manner as to cancel out the speed variation caused by the belt thickness inconsistency. As a result, the intermediate transfer belt can move at a stable speed.
In order to control the driving speed of the driving motor, it is necessary to determine a reference timing indicating when a predetermined reference portion of the intermediate transfer belt rotates once in the circumferential direction (endless movement direction). There is known a method of determining such a reference timing, performed with the use of a home position sensor. In this method, a home position mark is applied to the reference position of a belt member such as an intermediate transfer belt, and a home position sensor for detecting the home position mark is fixed at a predetermined position near the belt member. The reference timing is acquired as the home position sensor detects the home position mark.
Patent Document 1: Japanese Laid-Open Patent Application No. 2006-106642
However, this method incurs increased costs for applying a home position mark on the belt member and providing a home position sensor.
Accordingly, the inventors of the present invention are developing an image forming apparatus employing the following method for determining the reference timing without the use of a home position sensor. Specifically, as described above, the waveform of the speed variation pattern caused by the belt thickness inconsistency during one rotation is depicted by a sine curve corresponding to one period. The timing when a portion of a predetermined waveform appears, such as the maximum value, the minimum value, or the mean value of the sine curve, can be considered to be the reference timing when a virtual reference portion of the belt enters a virtual home position. During a printing job, the driving speed of the driving motor is adjusted to reduce speed variations of the belt member. The waveform of the speed variation pattern formed due to the belt thickness inconsistency can be extracted based on the difference between the belt speed detected by the encoder and the driving speed. The reference timing is specified based on the waveform thus extracted. Based on the specified reference timing, the driving speed of the belt is adjusted during the rotation of the belt beyond the reference timing. With such a configuration, the cost required for providing a home position sensor can be eliminated.
However, with a belt driving device having such a configuration, when the image carrier (for example, a photoconductor) and the belt member are separated from each other, the reference timing determined during this rotation may be significantly erroneous. Specifically, when a tandem type image forming apparatus is in a monochrome mode for forming monochrome images, usually only the K image carrier, among the Y, M, C, and K image carriers, contacts the belt member to perform the image forming operation. Furthermore, when new image information is received during an image forming operation, the image forming operation for the new image information is performed immediately after the previous image forming operation, without stopping the image carrier or the belt member. Assuming that the image forming apparatus with such a configuration uses the above-described virtual home position, the following problems arise. When color image information is received while performing the image forming operation in the monochrome mode, the following series of operations are performed. That is, when the image forming operation in the monochrome mode ends, the Y, M, and C photoconductors, which had been separated from the belt member, come in contact with the belt member, while the belt member is continuously driven. Accordingly, the image forming operation in the color mode starts while the belt member is continuously driven. During these operations, when the Y, M, and C photoconductors come in contact with the belt member, the load on the driving motor may rapidly increase, which may instantaneously significantly decrease the moving speed of the belt member. As a result, the waveform of the speed variation pattern may become considerably irregular. Accordingly, the image forming apparatus may erroneously detect a timing of a waveform, which does not correspond to the virtual reference portion of the belt member, as being the reference timing corresponding to when the reference portion enters the virtual home position. Such an erroneous detection causes a considerable error in determining the reference timing. Furthermore, when monochrome image information is received while an image forming operation is being performed in the color mode, the Y, M, and C image carriers which have been in contact with the belt member are separated from the belt member as the color mode image forming operation ends. In this case, the load on the driving motor may rapidly decrease, which may instantaneously significantly increase the moving speed of the belt member. Such an instantaneous increase in the moving speed may cause a considerable error in determining the reference timing.
When there is a considerable error in determining the reference timing of the belt member, the moving speed of the belt member cannot be properly stabilized in the subsequent rotation, which leads to considerable color shift.
The above describes the problems that arise in a configuration of making an image carrier come in contact with/separate from a belt member. The same problems arise when making any other opposing member facing the belt member come in contact with/separate from the belt member. For example, a belt cleaning device for cleaning off residual toner remaining on the surface of the belt member after the transfer process, or an opposing member such as a transfer member for contacting the surface of the belt member to form a transfer nip, may be configured to come in contact with/separate from the belt member, which leads to the same problems as described above.